Why you cant use QOU's in 48 Vdc Systems [RE-wrenches]

[jberdner at sma-america.com]

Wrenches:

The fundamental problem with the QOU's is the 50 Vdc rating.
In a 48 Vdc system the battery voltage is routinely above 50 Vdc.
During equalization you can get above 60 Vdc.
On the PV side the open circuit voltage can be ~ 110 = (4 * 22 * 1.25
"temperature derate")

At APT and then later at Pulse we went round and round with this one.
In the old APT power centers we were only able to use QOU's when UL had
Listed them with the Class T fuses in series.
This is why we switched to the Heinemann / Airpax E size breakers with
125 Vdc ratings when we went to the 4B design with 250 Amp breakers.

The bottom line is that a UL Listing trumps the manufacturers rating
because UL has run a bunch of extra tests.
They consider interrupt ratings, wire sizes, minimum lengths of wire,
etc. when they List a product above the manufacturer's ratings.
Without the special UL testing, you can't use QOU's as a 48 Vdc "general
purpose" load center.

If you have any questions or I can be of any further assistance please
do not hesitate to contact me.

Best regards,

John Berdner

SMA-America, Inc.
20830 Red Dog Road
Grass Valley, CA  95945
Tel: 530.273.4895
Fax: 530.274.7271


-----Original Message-----
From: Allan Sindelar [mailto:allan at positiveenergysolar.com]
Sent: Sunday, July 15, 2001 12:44 PM
To: RE-wrenches at topica.com
Subject: Re: 48V hardware [RE-wrenches]


William,
What am I missing here. My understanding is that they, like the QO, are
rated for 50VDC. A 48V nominal system is usually over 50V. How can I use
QUOs? Thanks, Allan
----- Original Message -----
From: William Miller
Sent: Saturday, July 14, 2001 8:57 AM
Allan:

Why don't you think the Square D QO is suitable? My conclusion is that
Square D is suitable up to 125 volts DC unless connected directly to a
battery bank, as it lacks only the AIC rating. I use QO with confidence
as long as it is isolated from the batteries by a fuse or a Heinman
breaker.

William Miller



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